Aqueous ink, ink jet recording method, ink cartridge, recording unit and ink jet recording apparatus

ABSTRACT

The invention provides an aqueous ink comprising a polymer having hydroxyl groups and a pigment, which can provide images excellent in scratch resistance and highlighter resistance and can inhibit the seeping out phenomenon of the polymer and the deterioration of storage stability and ejection characteristics even when it is stored for a long period of time. The aqueous ink comprises a polymer having hydroxyl groups and a pigment, wherein a proportion of a content of a polymer that is not adsorbed on the pigment out of the polymer having hydroxyl groups to the content of the pigment in the ink is 2.0 mass % or more, and a content (mass %) of the polymer that is not adsorbed on the pigment out of the polymer having hydroxyl groups is 0.20 mass % or less based on the total mass of the ink.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aqueous ink comprising a polymer anda pigment.

2. Description of the Related Art

It has been known to use a pigment as a coloring material of an ink inorder to make excellent the fastness properties, such as light fastness,gas fastness and water fastness, of an image obtained by an ink jetrecording method. It has also been known to use in an ink a resindispersion pigment that is dispersed with a polymer as a dispersant, ora self-dispersible pigment to the surface of which an hydrophilic groupis bonded and a water-soluble polymer in order to improve the scratchresistance and highlighter resistance of an image (Japanese PatentApplication Laid-Open No. H05-179183).

SUMMARY OF THE INVENTION

The present inventors have carried out a detailed investigation as toinks comprising a resin dispersion pigment with a view toward improvingthe fastness properties, such as light fastness, gas fastness and waterfastness, of an image obtained by an ink jet recording method as well asthe scratch resistance and highlighter resistance thereof. As a result,it has been found that the ejection characteristics of the inks aregreatly affected by the type or properties of a polymer functioning as adispersant.

Thus, the present inventors have carried out a further detailedinvestigation as to polymers used as dispersants for the purpose ofmaking excellent the fastness properties, scratch resistance andhighlighter resistance of images as well as the ejection characteristicsof inks. As a result, it has been found that an ink in which a pigmentis dispersed with a polymer having hydroxyl groups can solve theabove-described problems.

However, it has been found that when an ink cartridge stored thereinwith an ink in which a pigment is dispersed with a polymer havinghydroxyl groups is installed in an ink jet recording apparatus and thenis left to stand for a long period of time without ejecting the ink, thefollowing new technical problem is raised. Namely, it has been foundthat the polymer in the ink seeps out through ejection orifices of arecording head, adheres in the vicinity of the ejection orifices andthen sticks there, thereby the ejection characteristics aredeteriorated.

Accordingly, it is an object of the present invention to provide anaqueous ink (hereinafter may also be referred to as “ink”) capable ofsolving the following problems when the aqueous ink comprising a resindispersion pigment that is dispersed with a polymer having hydroxylgroups as a dispersant. In other words, the object is to provide anaqueous ink that can provide images excellent in scratch resistance andhighlighter resistance and can inhibit the seeping out phenomenon of thepolymer and the deterioration of ejection characteristics even when itis stored for a long period of time.

Another object of the present invention is to provide an ink cartridge,a recording unit and an ink jet recording method using theabove-described aqueous ink.

The above objects can be achieved by the present invention describedbelow. More specifically, an aqueous ink according to the presentinvention comprises a polymer having hydroxyl groups and a pigment,wherein a proportion of a content of a polymer that is not adsorbed onthe pigment out of the polymer having hydroxyl groups to a content ofthe pigment in the ink is 2.0 mass % or more, and wherein a content(mass %) of the polymer that is not adsorbed on the pigment out of thepolymer having hydroxyl groups is 0.20 mass % or less based on the totalmass of the ink.

The ink jet recording method according to another embodiment of thepresent invention is an ink jet recording method comprising ejecting anink by an ink jet method to conduct recording on a recording medium,wherein the ink is the aqueous ink of the above-described constitution.

The ink cartridge according to a further embodiment of the presentinvention is an ink cartridge comprising an ink storage portion storingan ink, wherein the ink is the aqueous ink of the above-describedconstitution.

The recording unit according to a still further embodiment of thepresent invention is a recording unit comprising an ink storage portionstoring an ink and a recording head for ejecting the ink, wherein theink is the aqueous ink of the above-described constitution.

The ink jet recording apparatus according to a yet still furtherembodiment of the present invention is an ink jet recording apparatuscomprising an ink storage portion storing an ink and a recording headfor ejecting the ink, wherein the ink is the aqueous ink of theabove-described constitution.

According to the present invention, there can be provided an aqueous inkcapable of solving the following problems when using the aqueous inkcomprising a resin dispersion pigment that is dispersed with a polymerhaving hydroxyl groups as a dispersant. In other words, there can beprovided an aqueous ink that can provide images excellent in scratchresistance and highlighter resistance and can inhibit the seeping outphenomenon of the polymer and the deterioration of storage stability andejection characteristics even when it is stored for a long period oftime.

According to another embodiment of the present invention, there can beprovided an ink cartridge, a recording unit and an ink jet recordingmethod using the above-described aqueous ink.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C typically illustrate the condition that the state ofan ink changes with time within a nozzle.

FIG. 2 is a longitudinal cross-sectional view illustrating a recordinghead.

FIG. 3 is a transverse cross-sectional view of the recording head.

FIG. 4 is a perspective view of a multi-head composed of an array of anumber of recording heads as shown in FIG. 3.

FIG. 5 is a perspective view illustrating an exemplary ink jet recordingapparatus.

FIG. 6 is a longitudinal cross-sectional view illustrating an inkcartridge.

FIG. 7 is a perspective view illustrating an exemplary recording unit.

FIG. 8 typically illustrates the construction of an exemplary recordinghead.

DESCRIPTION OF THE EMBODIMENTS

The present invention will hereinafter be described in more detail bythe best mode for carrying out the invention.

The present inventors have carried out a detailed investigation as toinks comprising a resin dispersion pigment with a view toward improvingthe scratch resistance and highlighter resistance of images obtained byan ink jet recording method. The inventors have supposed that theejection characteristics can be improved by enhancing the hydrophilicityof a pigment, and thus carried out an investigation as to the use of apolymer having hydroxyl groups as a dispersant for the resin dispersionpigment further. However, it has been found that when an ink cartridgestored therein an ink containing a resin dispersion pigment dispersedwith a polymer having hydroxyl groups is installed in an ink jetrecording apparatus and then is left to stand for a long period of timewithout ejecting the ink, the following problem is raised. In otherwords, it has been confirmed that a phenomenon that the polymer in theink seeps out through ejection orifices of a recording head occurs.Thus, the present inventors have carried out an investigation as tocauses of the above phenomenon. As a result, the inventors haveconcluded that the following phenomenon caused within a nozzle of therecording head is one of the causes. The mechanism with which thepolymer seeps out will hereinafter be described in detail with referenceto FIGS. 1A, 1B and 1C.

FIGS. 1A, 1B and 1C typically illustrate the condition that the state ofan ink containing a resin dispersion pigment dispersed by a polymerhaving hydroxyl groups changes with time within a nozzle.

FIG. 1A illustrates a state after the ink has been ejected from anejection orifice 1 by thermal energy applied from a heater 2. Asillustrated in FIG. 1A, water molecule 4, pigment particle 5 on whichthe polymer has been adsorbed, a water-soluble organic solvent 6, and apolymer having hydroxyl groups that is not adsorbed on the pigment (afree polymer 7 having hydroxyl groups) are present within the nozzle ina uniformly dispersed state. Incidentally, in the present invention, apolymer that is not adsorbed on the pigment is referred to as “freepolymer”, and a polymer having hydroxyl groups in a state not adsorbedon a pigment is referred to as “free polymer having hydroxyl groups”.

FIG. 1B illustrates a state after left to stand for a certain period oftime with the state of FIG. 1A. As illustrated in FIG. 1B, water firstevaporates from the ejection orifice 1 within the nozzle with time(water molecules 8 evaporated). When the time has further elapsed, thewater molecule 4 within the nozzle transfers in the direction of theejection orifice 1. At this time, the free polymer 7 having hydroxylgroups, which is forming a hydrogen bond with the water molecule 4, alsotransfers together with the water molecule 4 in the direction of theejection orifice 1.

FIG. 1C illustrates a state after the time has still further elapsed. Asillustrated in FIG. 1C, the free polymer 7 having hydroxyl groups, whichis forming a hydrogen bond with the water molecule 4, seeps out throughthe ejection orifice 1 and adheres to an orifice face 3 centering on theejection orifice 1. The free polymer 7 having hydroxyl groups rightafter having adhered to the orifice face 3 keeps a dissolved state bythe bonded water molecule 4. Since the bonded water molecule 4evaporates with time, however, the free polymer 7 having hydroxyl groupsloses its solubility to stick as a deposit on the periphery of theejection orifice 1.

The phenomenon that the polymer in the ink seeps out through theejection orifice of the recording head occurs according to theabove-described mechanism. As a result, it is considered that theejection characteristics are deteriorated by the presence of thedeposit.

Incidentally, whether all the free polymer having hydroxyl groupspresent in the ink according to the present invention forms the hydrogenbond with the water molecules or nor is unknown. It is however inferredthat majority of the polymer having hydroxyl groups is present in thestate of forming a hydrogen bond with the water molecule in order tomore stably be present in the ink.

As described above, the polymer to cause seeping out is a polymer havinghydroxyl groups that is not adsorbed on the pigment, i.e., a freepolymer having hydroxyl groups. As a consequence, it is supposed thatthe deterioration of the ejection characteristics can be inhibited byreducing the content of the free polymer having hydroxyl groups in theink.

The present inventors have thus carried out an investigation with a viewtoward reducing the free polymer having hydroxyl groups in the ink tothe utmost within such a range that the ejection characteristics are notdeteriorated and the scratch resistance and highlighter resistance aresufficiently achieved. Specifically, an investigation has been made asto optimization of the content of the dispersant, the performance ofphysical means such as ultrafiltration and centrifugation, and the like.A further investigation has been made about the condition thatadsorbability between the pigment and the dispersant is enhanced bymaking the hydrophobicity of monomers making up the dispersant higher toprevent the generation of the free polymer having hydroxyl groups to theutmost.

As a result, it has been found that when the content (mass %) of thefree polymer having hydroxyl groups present in an ink is 0.20 mass % orless based on the total mass of the ink, such seeping out phenomenon asdescribed above can be inhibited, and the ejection characteristics canbe made excellent.

The present inventors have carried out a further investigation. As aresult, it has been found that when an ink, in which the content (mass%) of the free polymer having hydroxyl groups is 0.20 mass % or lessbased on the total mass of the ink, is stored for a long period of time,the following problem occurs. More specifically, it has been found thatgeneration of aggregates and changes of physical properties such assurface tension and viscosity may be caused in some cases, i.e., storagestability may be deteriorated.

The present inventors infer that the cause of such a phenomenon asdescribed above is as follows. When an ink using, as a dispersant for apigment, a polymer high in hydrophilicity like a polymer having hydroxylgroups is stored for a long period of time, there is a tendency toincrease the polymer that is desorbed from the surface of the pigmenthigh in hydrophobicity to be present in an aqueous medium high inhydrophilicity compared with the polymer that is present in a stateadsorbed on the surface of the pigment. Therefore, when the ink using,as a dispersant, the polymer having hydroxyl groups is stored for a longperiod of time, the polymer becomes easy to be desorbed from the surfaceof the pigment compared with a polymer having no hydroxyl group.

For inks using a conventional resin dispersion pigment, no investigationhas been made about the free polymer having hydroxyl groups in the inkand the content thereof, and hence the content of the free polymerhaving hydroxyl groups has not been controlled. As a result,adsorption/desorption between the pigment and the polymer takes place,since another free polymer having hydroxyl groups originally present inthe ink is adsorbed again on the surface of the pigment even when thepolymer is desorbed from the surface of the pigment, whereby the inkdoes not lose dispersion stability.

However, as described above, the free polymer having hydroxyl groupsneeds to be reduced to the utmost for inhibiting the seeping outphenomenon. It is thus necessary to remove the free polymer havinghydroxyl groups in the ink by ultrafiltration or the like. On one hand,this condition can be regarded as being such that it is hard to causethe phenomenon that the free polymer having hydroxyl groups is adsorbedagain on the surface of the pigment after the polymer is desorbed fromthe surface of the pigment. It is inferred that this resultingconflicting conditions cause deterioration of the storage stability ofthe ink using the polymer having hydroxyl groups as a dispersant whenthe ink is stored for a long period of time.

The present inventors have thus carried out an investigation as to thecomposition of an ink that can inhibit the seeping out phenomenon anddoes not cause the deterioration of storage stability when the ink isstored for a long period of time. In particular, the present inventorshave carried out an investigation as to the content of a free polymerhaving hydroxyl groups and the relationship between the contents of thefree polymer having hydroxyl groups and the pigment.

As a result, it has been found that the above-described problems can besolved at the same time in the following case, thus leading tocompletion of the present invention. That is, it has been found that theabove-described problems can be solved at the same time when the contentof the free polymer having hydroxyl groups is 0.20 mass % or less basedon the total mass of the ink, and the proportion of the content of thefree polymer having hydroxyl groups to the content of the pigment is 2.0mass % or more. Incidentally, in order to more effectively inhibit theseeping out phenomenon, it is more preferable that the content of thefree polymer having hydroxyl groups be 0.15 mass % or less based on thetotal mass of the ink. It is also preferable from the viewpoint ofstorage stability that the content (mass %) of the free polymer havinghydroxyl groups be 0.08 mass % or more based on the total mass of theink and the proportion of the content of the free polymer havinghydroxyl groups to the content of the pigment be 5.0 mass % or less.Incidentally, the content of the free polymer having hydroxyl groups inthe ink can be controlled by ultrafiltration or the like. <Ink>

The components constituting an ink according to the present inventionwill hereinafter be described.

(Polymer Having Hydroxyl Groups)

It is essential for the ink according to the present invention tocontain a polymer having hydroxyl groups. Polymers having hydroxylgroups include polymers obtained by copolymerizing, for example, amonomer having hydroxyl group(s) with a part of acrylic ester monomersamong acrylic ester polymers obtained by copolymerizing acrylic estermonomers.

Examples of the monomer having hydroxyl group(s) include 2-hydroxyethyl(meth)acrylate, hydroxypropyl (meth)acrylate, polyethylene glycolmono(meth)acrylate and polypropylene glycol mono(meth)acrylate.

As the monomer copolymerized with the monomer having hydroxyl group(s),may be used that commonly used. Examples of the acrylic ester monomerinclude methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate and cyclohexyl(meth)acrylate. Besides the acrylic ester monomers, any monomer having adouble bond copolymerizable with the monomer having hydroxyl group(s)may also be used. Examples thereof include a styrene monomer, vinylacetate monomer and 1,3-butadiene.

As the polymer having hydroxyl groups, may also be preferably used apolymer obtained by saponifying a vinyl acetate-acrylic ester polymerobtained by copolymerizing an acrylic ester monomer and a vinyl acetatemonomer with a strong base such as sodium hydroxide, i.e., a polyvinylalcohol type polymer.

No particular limitation is imposed on the form of the polymer so far asthe scratch resistance and highlighter resistance of an image as well asthe ejection characteristics of the ink can be made excellent, which isthe fundamental object to be achieved by the addition of the polymerhaving hydroxyl groups into an ink, and any polymer of linear, branched,random and block copolymers may be used. In order to inhibit aphenomenon that the polymer is desorbed from the surface of a pigment tothe utmost, however, the molecular weight of the segment having hydroxylgroups is preferably 20,000 or less.

When the polymer having hydroxyl groups is a copolymer composed of asegment having hydroxyl groups and a segment having no hydroxyl group,such a polymer is preferably a block copolymer, more preferably a blockcopolymer in which a monomer having hydroxyl group(s) is present at aterminal of the polymer, because the adsorbability of the polymer havinghydroxyl groups on the surface of the pigment is enhanced when thehydrophobic portion of the polymer having hydroxyl groups, which isadsorbed on the surface of the pigment, is concentrated on the terminal.When the ink according to the present invention is applied to an ink jetrecording method in which an ink is ejected by applying thermal energyto the ink to form an image on a recording medium, the ink tends to showmore stable ejection characteristics as the hydrophilicity of thepolymer is higher. From this fact, it is particularly preferable to usethe polyvinyl alcohol type polymer.

No particular limitation is imposed on the content (mass %) of thepolymer having hydroxyl groups in the ink according to the presentinvention so far as the scratch resistance and highlighter resistance ofan image as well as the ejection characteristics of the ink can be madeexcellent, which is the fundamental object. In the present invention,however, the content of the free polymer having hydroxyl groups, whichis the cause of the seeping out phenomenon, is preferably reduced tosuch an extent that the dispersion stability is not impaired.Accordingly, the content (mass %) of the polymer having hydroxyl groupsin the ink is preferably from 5.0% or more to 100.0% or less, morepreferably from 7.0% or more to less than 20.0%, further more preferablyfrom 10.0% or more to 15.0% or more based on the total mass of the inkwhen the content (mass %) of the pigment in the ink is regarded as 100based on the total mass of the ink. The content (mass %) of the polymerhaving hydroxyl groups is preferably less than 1.0 mass %, morepreferably less than 0.8 mass %, further more preferably less than 0.6mass % based on the total mass of the ink.

Incidentally, as for the resin dispersion pigment, the polymer is onlyphysically adsorbed on the surface of the pigment, so that the polymeris easy to be desorbed from the surface of the pigment compared with amicrocapsule type pigment or a pigment of the type that a polymer ischemically bonded to the surface of the pigment. Accordingly, in orderto retain storage stability when the ink is stored for a long period oftime, the content (mass %) of the free polymer having hydroxyl groups ispreferably 40.0% or more, more preferably 45.0% or more, based on thetotal content (mass %) of the polymers contained in the ink.

In the present invention, the content of the free polymer havinghydroxyl groups in the ink somewhat varies according to the structure ofthe polymer and the pigment used together with the polymer, and thelike. In this case, however, the content may be suitably controlled byultrafiltration or the like.

(Content of Free Polymer Having Hydroxyl Groups in Ink)

In the present invention, the content of the free polymer havinghydroxyl groups in the ink can be measured in accordance with thefollowing method. The present invention is not limited to the followingmethod, and a value measured by any other method may be used.

An ink is centrifuged under conditions of 400,000 G and 16 hours, 95mass % of a liquid in a supernatant portion is taken out. Thereafter,the resultant liquid is subjected to acid dipping to take out a freepolymer. The resultant free polymer is then dried to solid to determinethe amount of the solid matter. The content of the free polymer based onthe total mass of the ink is then found from the proportion to thecharged amount. In this method, fine pigment particles may be containedin the liquid of the supernatant portion taken out after thecentrifugation under the above-described conditions in some cases. Inthe present invention, however, the solid matter obtained by conductingthe acid dipping by the above-described procedure is substantiallyregarded as solid matter of the free polymer because the amount of thepigment contained is very small. When the polymer in the ink iscomprised of only the polymer having hydroxyl groups, the solid matterof this free polymer is solid matter of the free polymer having hydroxylgroups, and the content (mass %) of the free polymer having hydroxylgroups based on the total mass of the ink is found from this value.

Incidentally, the ink may contain two or more free polymers, forexample, the free polymer having hydroxyl groups and a free polymerhaving no hydroxyl group. In such a case, it is necessary to make thefollowing arrangement in order to determine the content of the freepolymer having hydroxyl groups. Namely, the polymer having hydroxylgroups is taken out by using MALDI-TOF/MS (a combination of matrixassisted laser desorption ionization and time-of-flight massspectrometry), arranging the acid dipping conditions or conducting GPC.The obtained polymer is subjected to component analysis as to monomersconstituting the polymer by means of ¹³C-NMR, gas chromatography and/orthe like. An 1 mass % aqueous solution of a polymer having the samestructure as the polymer contained in the ink (standard sample) isprepared on the basis of this analytical result. Thereafter, an aqueoussolution containing 1 mass % of the free polymer obtained above and thestandard sample are subjected to infrared absorption spectrometry underthe same conditions. From the result of the infrared absorptionspectrometry as to the standard sample and the aqueous solutioncontaining the free polymer, mass conversion is conducted to find thecontent (mass %) of the free polymer having hydroxyl groups based on thetotal mass of the ink.

(Pigment)

Examples of pigments usable in the inks according to the presentinvention include carbon black and organic pigments. The content (mass%) of the pigment is preferably from 0.1 mass % or more to 10.0 mass %or less based on the total mass of the ink.

[Carbon Black]

Examples of usable carbon black include carbon black pigments such asfurnace black, lamp black, acetylene black and channel black. It goeswithout saying that carbon black pigments are not limited to thefollowing pigments.

As specific examples thereof, may be mentioned Raven: 1170, 1190ULTRA-II, 1200, 1255, 1500, 2000, 3500, 5000, 5250, 5750 and 7000 (all,products of Columbian Carbon Japan Limited); Black Pearls L, Regal:330R, 400R and 660R, Mogul L, Monarch: 700, 800, 880, 900, 1000, 1100,1300 and 1400, and Valcan XC-72R (all, products of CABOT CO.); ColorBlack: FW1, FW2, FW2V, FW18, FW200, S150, S160 and S170, Printex: 35, U,V, 140U and 140V, and Special Black: 4, 4A, 5 and 6 (all, products ofDegussa AG); and No. 25, No. 33, No. 40, No. 47, No. 52, No. 900, No.2300, MCF-88, MA7, MA8, MA100 and MA600 (all, products of MITSUBISHICHEMICAL CORPORATION).

Besides the above pigments, any carbon black conventionally known may beused. Magnetic fine particles such as magnetite and ferrite, titaniumblack, and the like may also be used as black pigments.

[Organic Pigment]

The organic pigments specifically include the following pigments, suchas insoluble azo pigments such as Toluidine Red, Toluidine Maroon, HansaYellow, Benzidine Yellow and Pyrazolone Red; soluble azo pigments suchas Lithol Red, Helio Bordeaux, Pigment Scarlet and Permanent Red 2B;derivatives from vat dyes, such as alizarin, indanthron and ThioindigoMaroon; phthalocyanine pigments such as Phthalocyanine Blue andPhthalocyanine Green; quinacridone pigments such as Quinacridone Red andQuinacridone Magenta; perylene pigments such as Perylene Red andPerylene Scarlet; isoindolinone pigments such as Isoindolinone Yellowand Isoindolinone Orange; imidazolone pigments such as BenzimidazoloneYellow, Benzimidazolone Orange and Benzimidazolone Red; pyranthronepigments such as Pyranthrone Red and Pyranthrone Orange; indigopigments; condensed azo pigments; thioindigo pigments; FlavanthroneYellow; Acylamide Yellow; Quinophthalone Yellow; Nickel Azo Yellow;Copper Azomethine Yellow; Perinone Orange; Anthrone Orange;Dianthraquinonyl Red; Dioxazine Violet; etc.

When organic pigments are indicated by COLOR INDEX (C.I.) numbers, thefollowing pigments may be exemplified. It goes without saying thatconventionally known organic pigments may also be used in addition tothe following pigments.

C.I. Pigment Yellow: 12, 13, 14, 17, 20, 24, 74, 83, 86, 93, 109, 110,117, 120, 125, 128, 137, 138, 147, 148, 151, 153, 154, 166 and 168; C.I.Pigment Orange: 16, 36, 43, 51, 55, 59 and 61; C.I. Pigment Red: 9, 48,49, 52, 53, 57, 97, 122, 123, 149, 168, 175, 176, 177, 180, 192, 215,216, 217, 220, 223, 224, 226, 227, 228, 238 and 240; C.I. PigmentViolet: 19, 23, 29, 30, 37, 40 and 50; C.I. Pigment Blue: 15, 15:3,15:1, 15:4, 15:6, 22, 60 and 64; C.I. Pigment Green: 7 and 36; and C.I.Pigment Brown 23, 25 and 26.

(Dispersant)

In the inks according to the present invention, it is essential to usethe polymer having hydroxyl groups as a dispersant in consideration ofscratch resistance, highlighter resistance and ejection characteristics.Further, any other polymer may be used as a dispersant in the inksaccording to the present invention so far as the effect by adding themis achieved, and the objects and effects of the present invention arenot impaired.

As another dispersant, is preferably used that capable of stablydispersing the pigment in an aqueous medium by an action of an anionicgroup. For example, the following dispersants may be used.

Specific examples thereof include styrene-acrylic acid copolymers,styrene-acrylic acid-alkyl acrylate terpolymers and salts thereof;styrene-maleic acid copolymers, styrene-maleic acid-alkyl acrylateterpolymers and salts thereof; styrene-methacrylic acid copolymers,styrene-methacrylic acid-alkyl acrylate terpolymers and salts thereof;styrene-maleic acid half ester copolymers, vinylnaphthalene-acrylic acidcopolymers, vinylnaphthalene-maleic acid copolymers, styrene-maleicanhydride-maleic acid half ester terpolymers and salts thereof.

The dispersant used in the present invention preferably has a weightaverage molecular weight ranging from 1,000 or more to 30,000 or less,more preferably from 3,000 or more to 15,000 or less.

(Aqueous Medium)

In the inks according to the present invention, an aqueous medium, whichis water, or a mixed solvent of water and any of various water-solubleorganic solvents, may be used.

No particular limitation is imposed on the water-soluble organic solventso far as it is soluble in water, and the following solvents may beused.

Polyhydric alcohols, in which hydroxyl groups are bonded to saturatedcarbon atoms, such as 1,5-pentanediol, 1,2-hexanediol, 1,6-hexanediol,1,2,6-hexanetriol, 1,1,1-trimethylolpropane and glycerol; monohydricalkyl alcohols having 1 to 4 carbon atoms, such as ethyl alcohol,n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcoholand tert-butyl alcohol; amides such as dimethylformamide anddimethylacetamide; ketones and ketone alcohols such as acetone anddiacetone alcohol; ethers such as tetrahydrofuran and dioxane;polyalkylene glycols such as polyethylene glycol and polypropyleneglycol; alkylene glycols the alkylene group of which has 2 to 6 carbonatoms, such as ethylene glycol, propylene glycol, butylene glycol,triethylene glycol and thiodiglycol; lower alkyl ether acetates such aspolyethylene glycol monomethyl ether acetate; bishydroxyethyl sulfone;lower alkyl glycol ethers such as ethylene glycol monomethyl (or ethyl,butyl) ether, diethylene glycol monomethyl (or ethyl, butyl) ether andtriethylene glycol monomethyl (or ethyl, butyl) ether; lower dialkylglycol ethers such as triethylene glycol dimethyl (or ethyl) ether andtetraethylene glycol dimethyl (or ethyl) ether; alkanolamines such asmonoethanolamine, diethanolamine and triethanolamine; sulfolane;N-methyl-2-pyrrolidone; 2-pyrrolidone; 1,3-dimethyl-2-imidazolidinone.

In the present invention, it is particularly preferable that the inkcontains polyethylene glycol having an average molecular weight of from600 or more to 1,500 or less, since ejection characteristics,particularly, ejection characteristics when the ink is continuouslyejected, can be made excellent. In particular, the content ofpolyethylene glycol having an average molecular weight of from 600 ormore to 1,500 or less is preferably 0.5 times or more as much as thecontent of the pigment (solid content) in the ink.

The content (mass %) of the water-soluble organic solvent is preferablyfrom 3.0 mass % or more to 50.0 mass % or less based on the total massof the ink.

As the water, it is preferable to use deionized water (ion-exchangedwater). The content (mass %) of water is preferably from 40.0 mass % ormore to 95.0 mass % or less based on the total mass of the ink.

(Other Additives)

Besides the above components, a surfactant, an antifoaming agent, apreservative, a mildewproofing agent and the like may be added to theinks used in the present invention, not to mention the addition of ahumectant, as needed, to obtain an ink having desired physicalproperties.

When the resin dispersion pigment is used as a coloring material likethe present invention, however, the surfactant tends to be adsorbed onthe surface of the pigment. Therefore, when a polymer high inhydrophilicity and easy to be desorbed from the surface of the pigmentlike the polymer having hydroxyl groups is used as a dispersant, it ispreferable to lessen the content of the surfactant in the ink to theutmost. Specifically, the content (mass %) of the surfactant in the inkis preferably 25% or less, more preferably 12.5% or less, when thecontent (mass %) of the pigment (solid content) in the ink is regardedas 100.

<Ink Jet Recording Method, Ink Cartridge, Recording Unit and Ink JetRecording Apparatus>

An exemplary ink jet recording apparatus will hereinafter be described.First of all, an exemplary construction of a recording head, which is amain component of the ink jet recording apparatus making good use ofthermal energy is shown in FIG. 3 and FIG. 4. FIG. 3 is across-sectional view of a recording head 13 taken along the flow path ofink, and FIG. 4 is a cross-sectional view taken along line 4-4 in FIG.3. The recording head 13 is formed by a member having a flow path(nozzle) 14, through which an ink is passed, and a heating elementsubstrate 15. The heating element substrate 15 is composed of aprotective layer 16, electrodes 17-1 and 17-2, a heating resistor layer18, a heat accumulating layer 19 and a substrate 20.

When pulsed electric signals are applied to the electrodes 17-1 and 17-2of the recording head 13, the heating element substrate 15 rapidlygenerates heat at the region shown by ‘n’ to form bubbles in an ink 21which is in contact with this region. A meniscus 23 of the ink isprojected by the pressure of the bubbles, and the ink 21 is ejected froman ejection orifice 22 through the nozzle 14 toward a recording medium25.

FIG. 5 illustrates an appearance of a multi-head composed of an array ofa number of heads as shown in FIG. 3. The multi-head is composed of aglass plate 27 having a number of nozzles 26 and a recording head 28similar to that described in FIG. 3.

FIG. 6 is a perspective view illustrating an exemplary ink jet recordingapparatus in which a recording head has been incorporated. A blade 61 isa wiping member, one end of which is a stationary end held by ablade-holding member to form a cantilever. The blade 61 is arranged at aposition adjacent to a region, in which a recording head 65 operates,and is held in a form protruding into the course through which therecording head 65 is moved.

Reference numeral 62 indicates a cap for the face of ejection orificesof the recording head 65, which is provided at a home position adjacentto the blade 61, and is so constructed that it moves in a directionperpendicular to a direction in which the recording head 65 is moved,and comes into contact with the face of the ejection orifices to cap it.Reference numeral 63 denotes an ink absorbing member providedadjoiningly to the blade 61 and, similar to the blade 61, held in a formprotruding into the course through which the recording head 65 is moved.An ejection-recovery portion 64 is constructed by the blade 61, cap 62and ink absorbing member 63. Water, dust and/or the like are removedfrom the face of the ink-ejecting orifices by the blade 61 and inkabsorbing member 63.

Reference numeral 65 designates the recording head having anejection-energy-generating means and serving to eject the ink on to arecording medium set in an opposing relation to the ejection orificeface provided with the ejection orifices to conduct recording. Referencenumeral 66 indicates a carriage on which the recording head 65 ismounted so that the recording head 65 can be moved. The carriage 66 isslidably interlocked with a guide shaft 67 and is connected (notillustrated) at its part to a belt 69 driven by a motor 68. Thus, thecarriage 66 can be moved along the guide shaft 67 and hence, therecording head 65 can be moved from a recording region to a regionadjacent thereto.

Reference numerals 51 and 52 denote a feeding part from which therecording medium is inserted, and feed rollers driven by a motor (notillustrated), respectively. With such a construction, the recordingmedium is fed to the position opposite to the ejection orifice face ofthe recording head 65, and discharged from a discharge section providedwith discharge rollers 53 with the progress of recording. The cap 62 inthe ejection-recovery portion 64 is receded from the path of motion ofthe recording head 65 when the recording head 65 is returned to its homeposition after completion of recording by the recording head 65, and theblade 61 remains protruded into the path of motion. As a result, theejection orifice face of the recording head 65 is wiped.

When the cap 62 comes into contact with the ejection orifice face of therecording head 65 to cap it, the cap 62 is moved so as to protrude intothe path of motion of the recording head 65. When the recording head 65is moved from its home position to the position at which recording isstarted, the cap 62 and blade 61 are at the same positions as thepositions for the wiping as described above. As a result, the ejectionorifice face of the recording head 65 is also wiped at the time of thismovement. The above movement of the recording head 65 to its homeposition is made not only when the recording is completed or therecording head 65 is recovered for ejection, but also when the recordinghead 65 is moved between recording regions for the purpose of recording,during which it is moved to the home position adjacent to each recordingregion at given intervals, where the ejection orifice face is wiped inaccordance with this movement.

FIG. 7 illustrates an exemplary ink cartridge in which an ink to be fedto a recording head through an ink-feeding member, for example, a tubeis stored. Here, reference numeral 40 designates an ink storage portionstoring the ink to be fed, as exemplified by a bag for the ink. One endthereof is provided with a stopper 42 made of rubber. A needle (notillustrated) may be inserted into this stopper 42 so that the ink in thebag 40 for the ink can be fed to the recording head. Reference numeral44 indicates an absorbing member for receiving a waste ink.

The ink jet recording apparatus are not limited to the apparatus inwhich the recording head and the ink cartridge are separately provided.Therefore, an apparatus in which these members are integrally formed asshown in FIG. 8 may also be preferably used. In FIG. 8, referencenumeral 70 designates a recording unit, in the interior of which an inkstorage portion storing an ink, for example, an ink absorbing member, iscontained, and the ink in the ink absorbing member is ejected in theform of ink droplets through a recording head 71 having a plurality ofejection orifices. The ink storage portion may be constructed by a bagfor the ink, in the interior of which a spring or the like is provided,without using the ink absorbing member. Reference numeral 72 indicatesan air passage for communicating the interior of the recording unit 70with the atmosphere. This recording unit 70 is used in place of therecording head 65 shown in FIG. 6, and is detachably installed on thecarriage 66.

An ink jet recording apparatus making good use of mechanical energy willnow be described. The apparatus is equipped with a nozzle-formingsubstrate having a plurality of nozzles, pressure-generating devicescomposed of a piezoelectric material and an electric conductivematerial, and an ink filled around the pressure-generating devices andfeatures a recording head in which the pressure-generating devices arechanged by applying a voltage to eject droplets of the ink from ejectionorifices. FIG. 9 typically illustrates an example of the construction ofthe recording head. The recording head is constructed by an ink flowpath 80 communicating with an ink chamber, an orifice plate 81, avibration plate 82 for applying a pressure to the ink, a piezoelectricelement 83 bonded to the vibration plate 82 undergoing a changeaccording to an electric signal, and a substrate 84 adapted to supportand fix the orifice plate 81, the vibration plate 82 and the likethereon. The vibration plate 82 bonded to the piezoelectric element 83is deformed by strain stress generated by applying a pulsed voltage tothe piezoelectric element 83 to pressurize the ink in the ink flow path80, thereby ejecting ink droplets from the ejection orifice 85 of theorifice plate 81. Such a recording head can be used by incorporating itinto an ink jet recording apparatus similar to that illustrated in FIG.6.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples. However, the presentinvention is not limited by these examples so far as the subject matterof the present invention is not overstepped. Incidentally, alldesignations of “part” or “parts” and “%” as will be used in thefollowing examples mean part or parts by mass and mass % unlessexpressly noted.

<Preparation of Pigment Dispersion Solution>

In the preparation of the following pigment dispersion solutions, thefollowing carbon black and polymers were used. However, the presentinvention is not limited thereto.

Carbon black: one having a specific surface area of 210 m²/g and a DBPoil absorption of 74 ml/100 g.

Polymer A: a polymer obtained by saponifying and neutralizing polyvinylacetate-b-poly (benzyl methacrylate-co-acrylic acid) (compositional(molar) ratio: 20:60:20) having a weight average molecular weight of8,000 with a 10 mass % aqueous solution of potassium hydroxide.

Polymer B: a polymer obtained by neutralizing poly(styrene-co-acrylicacid) (compositional (molar) ratio: 70:30) having a weight averagemolecular weight of 12,000 with a 10 mass % aqueous solution ofpotassium hydroxide.

Polymer C: a polymer obtained by neutralizing poly(2-hydroxyethylmethacrylate)-b-poly(benzyl methacrylate-co-acrylic acid) (compositional(molar) ratio: 20:60:20) having a weight average molecular weight of8,000 with a 10 mass % aqueous solution of potassium hydroxide.

Polymer D: a polymer obtained by saponifying and neutralizing poly(vinylacetate-co-benzyl methacrylate-co-acrylic acid) (compositional (molar)ratio: 20:60:20) having a weight average molecular weight of 8,000 witha 10 mass % aqueous solution of potassium hydroxide.

(Preparation of Black Pigment Dispersion Solution 1)

Ten parts of carbon black, 1 part of Polymer A and 89 parts ofion-exchanged water were mixed and dispersed for 3 hours by means of abatch type vertical sand mill. The resultant dispersion solution wascentrifuged, thereby removing coarse particles, and then filtered underpressure through a microfilter (product of Fuji Photo Film Co., Ltd.)having a pore size of 3.0 μm, thereby preparing Resin Dispersion BlackPigment 1. Water was added to Resin Dispersion Black Pigment 1 todisperse the pigment so as to give a pigment concentration of 10 mass %,thereby preparing a dispersion solution to obtain Black PigmentDispersion Solution 1 in accordance with the above-described process.The resultant Black Pigment Dispersion Solution 1 was subjected toultrafiltration to adjust the content of the free polymer havinghydroxyl groups. The mass ratio (pigment:Polymer A) of the content ofthe pigment to the content of Polymer A in Black Pigment DispersionSolution 1 was 100:8.

(Preparation of Black Pigment Dispersion Solution 2)

Ten parts of carbon black, 2 parts of Polymer A, 2 parts of Polymer Band 86 parts of ion-exchanged water were mixed and dispersed for 3 hoursby means of a batch type vertical sand mill. The resultant dispersionsolution was centrifuged, thereby removing coarse particles, and thenfiltered under pressure through a microfilter (product of Fuji PhotoFilm Co., Ltd.) having a pore size of 3.0 μm, thereby preparing ResinDispersion Black Pigment 2. Water was added to Resin Dispersion BlackPigment 2 to disperse the pigment so as to give a pigment concentrationof 10 mass %, thereby preparing a dispersion solution to obtain BlackPigment Dispersion Solution 2 in accordance with the above-describedprocess. The resultant Black Pigment Dispersion Solution 2 was subjectedto ultrafiltration to adjust the content of the free polymer havinghydroxyl groups. In order to find the mass ratio of the content of thepigment to the content of Polymer A in Black Pigment Dispersion Solution2, the following process was performed. After Black Pigment DispersionSolution 2 obtained above was dried to solid, the solid was dissolved intetrahydrofuran to obtain a solution. Thereafter, the resultant solutionwas filtered under pressure through a microfilter (product of Fuji PhotoFilm Co., Ltd.) having a pore size of 3.0 μm to separate the solutioninto the pigment and a solution containing the polymers. The solutioncontaining the polymers was subjected to GPC (gel permeationchromatography) to separately take out Polymer A and Polymer B. The massratio (pigment:Polymer A) of the content of the pigment to the contentof Polymer A in Black Pigment Dispersion Solution 2 was 100:7.

(Preparation of Black Pigment Dispersion Solution 3)

Ten parts of carbon black, 1 part of Polymer C and 89 parts ofion-exchanged water were mixed and dispersed for 3 hours by means of abatch type vertical sand mill. The resultant dispersion solution wascentrifuged, thereby removing coarse particles, and then filtered underpressure through a microfilter (product of Fuji Photo Film Co., Ltd.)having a pore size of 3.0 μm, thereby preparing Resin Dispersion BlackPigment 3. Water was added to Resin Dispersion Black Pigment 3 todisperse the pigment so as to give a pigment concentration of 10 mass %,thereby preparing a dispersion solution to obtain Black PigmentDispersion Solution 3 in accordance with the above-described process.The resultant Black Pigment Dispersion Solution 3 was subjected toultrafiltration to adjust the content of the free polymer havinghydroxyl groups. The mass ratio (pigment:Polymer C) of the content ofthe pigment to the content of Polymer C in Black Pigment DispersionSolution 3 was 100:9.

(Preparation of Black Pigment Dispersion Solution 4)

Ten parts of carbon black, 1 part of Polymer A and 89 parts ofion-exchanged water were mixed and dispersed for 4 hours by means of abatch type vertical sand mill. The resultant dispersion solution wascentrifuged, thereby removing coarse particles, and then filtered underpressure through a microfilter (product of Fuji Photo Film Co., Ltd.)having a pore size of 3.0 μm, thereby preparing Resin Dispersion BlackPigment 4. Water was added to Resin Dispersion Black Pigment 4 todisperse the pigment so as to give a pigment concentration of 10 mass %,thereby preparing a dispersion solution to obtain Black PigmentDispersion Solution 4 in accordance with the above-described process.The resultant Black Pigment Dispersion Solution 4 was subjected toultrafiltration to adjust the content of the free polymer havinghydroxyl groups. The mass ratio (pigment:Polymer A) of the content ofthe pigment to the content of Polymer A in Black Pigment DispersionSolution 4 was 100:9.

(Preparation of Black Pigment Dispersion Solution 5)

Ten parts of carbon black, 2 parts of Polymer C and 88 parts ofion-exchanged water were mixed and dispersed for 2 hours by means of abatch type vertical sand mill. The resultant dispersion solution wascentrifuged, thereby removing coarse particles, and then filtered underpressure through a microfilter (product of Fuji Photo Film Co., Ltd.)having a pore size of 3.0 μm, thereby preparing Resin Dispersion BlackPigment 5. Water was added to Resin Dispersion Black Pigment 5 todisperse the pigment so as to give a pigment concentration of 10 mass %,thereby preparing a dispersion solution to obtain Black PigmentDispersion Solution 5 in accordance with the above-described process.The resultant Black Pigment Dispersion Solution 5 was subjected toultrafiltration to adjust the content of the free polymer havinghydroxyl groups. The mass ratio (pigment:Polymer C) of the content ofthe pigment to the content of Polymer C in Black Pigment DispersionSolution 5 was 100:15.

(Preparation of Black Pigment Dispersion Solution 6)

Ten parts of carbon black, 1 part of Polymer D and 89 parts ofion-exchanged water were mixed and dispersed for 4 hours by means of abatch type vertical sand mill. The resultant dispersion solution wascentrifuged, thereby removing coarse particles, and then filtered underpressure through a microfilter (product of Fuji Photo Film Co., Ltd.)having a pore size of 3.0 μm, thereby preparing Resin Dispersion BlackPigment 6. Water was added to Resin Dispersion Black Pigment 6 todisperse the pigment so as to give a pigment concentration of 10 mass %,thereby preparing a dispersion solution to obtain Black PigmentDispersion Solution 6 in accordance with the above-described process.The resultant Black Pigment Dispersion Solution 6 was subjected toultrafiltration under the same conditions as in Black Pigment DispersionSolution 4 to adjust the content of the free polymer having hydroxylgroups. The mass ratio (pigment:Polymer D) of the content of the pigmentto the content of Polymer D in Black Pigment Dispersion Solution 6 was100:9.

(Preparation of Black Pigment Dispersion Solution 7)

Ten parts of carbon black, 2.5 parts of Polymer C and 87.5 parts ofion-exchanged water were mixed and dispersed for 2 hours by means of abatch type vertical sand mill. The resultant dispersion solution wascentrifuged, thereby removing coarse particles, and then filtered underpressure through a microfilter (product of Fuji Photo Film Co., Ltd.)having a pore size of 3.0 μm, thereby preparing Resin Dispersion BlackPigment 7. Water was added to Resin Dispersion Black Pigment 7 todisperse the pigment so as to give a pigment concentration of 10 mass %,thereby preparing a dispersion solution to obtain Black PigmentDispersion Solution 7 in accordance with the above-described process.The resultant Black Pigment Dispersion Solution 7 was subjected toultrafiltration under the same conditions as in Black Pigment DispersionSolution 5 to adjust the content of the free polymer having hydroxylgroups. The mass ratio (pigment:Polymer C) of the content of the pigmentto the content of Polymer C in Black Pigment Dispersion Solution 7 was100:16.

(Preparation of Black Pigment Dispersion Solution 8)

Ten parts of carbon black, 1 part of Polymer C and 89 parts ofion-exchanged water were mixed and dispersed for 3 hours by means of abatch type vertical sand mill. The resultant dispersion solution wascentrifuged, thereby removing coarse particles, and then filtered underpressure through a microfilter (product of Fuji Photo Film Co., Ltd.)having a pore size of 3.0 μm, thereby preparing Resin Dispersion BlackPigment 8. Water was added to Resin Dispersion Black Pigment 8 todisperse the pigment so as to give a pigment concentration of 10 mass %,thereby preparing a dispersion solution to obtain Black PigmentDispersion Solution 8 in accordance with the above-described process.The resultant Black Pigment Dispersion Solution 8 was subjected toultrafiltration in such a manner that the content of the free polymerhaving hydroxyl groups is less than that in Black Pigment DispersionSolution 3, thereby adjusting the content of the free polymer havinghydroxyl groups. The mass ratio (pigment:Polymer C) of the content ofthe pigment to the content of Polymer C in Black Pigment DispersionSolution 8 was 100:9.

(Preparation of Black Pigment Dispersion Solution 9)

Ten parts of carbon black, 3 parts of Polymer A and 87 parts ofion-exchanged water were mixed and dispersed for 3 hours by means of abatch type vertical sand mill. The resultant dispersion solution wascentrifuged, thereby removing coarse particles, and then filtered underpressure through a microfilter (product of Fuji Photo Film Co., Ltd.)having a pore size of 3.0 μm, thereby preparing Resin Dispersion BlackPigment 9. Water was added to Resin Dispersion Black Pigment 9 todisperse the pigment so as to give a pigment concentration of 10 mass %,thereby preparing a dispersion solution to obtain Black PigmentDispersion Solution 9 in accordance with the above-described process.The resultant Black Pigment Dispersion Solution 9 was subjected toultrafiltration under the same conditions as in Black Pigment DispersionSolution 1 to adjust the content of the free polymer having hydroxylgroups. The mass ratio (pigment:Polymer A) of the content of the pigmentto the content of Polymer A in Black Pigment Dispersion Solution 9 was100:17.

<Preparation of Ink>

After the components shown in the following Table 1 and Table 2 weremixed and sufficiently stirred, the resultant respective mixtures werefiltered under pressure through a microfilter (product of Fuji PhotoFilm Co., Ltd.) having a pore size of 1.0 μm, thereby preparing Inks 1to 12.

<Quantitative Determination of Free Polymer Having Hydroxyl Groups>

The content (mass %) of the free polymer having hydroxyl groups in eachink was determined in accordance with the following method. The resultsare shown in Table 1 and Table 2.

The ink was centrifuged under conditions of 400,000 G and 16 hours, and95 mass % of the liquid in the supernatant portion was taken out.Thereafter, the resultant liquid was subjected to acid dipping to takeout the free polymer. The resultant free polymer was dried to solid, andthe amount of the solid was determined. The content of the free polymerbased on the total mass of the ink was then found from the proportion tothe charged amount.

Incidentally, with respect to Ink 3 containing 2 polymers in the ink,the content of the free polymer having hydroxyl groups in the ink wasdetermined by performing the following process in addition to theabove-described procedure. Namely, the structure of Polymer A wasdetermined by a method known per se in the art, and a 1% aqueoussolution of Polymer A (standard sample) was prepared. Thereafter, anaqueous solution containing 1 mass % of the free polymer obtained aboveand the standard sample were subjected to infrared absorptionspectrometry under the same conditions. From the result of the infraredabsorption spectrometry as to the standard sample and the aqueoussolution containing the free polymer, mass conversion was conducted tofind the content (%) of the free polymer having hydroxyl groups based onthe total mass of the ink. TABLE 1 Ink 1 2 3 4 5 6 Coloring materialBlack Pigment Dispersion Solution 1 40.0 40.0 30.0 Black PigmentDispersion Solution 2 40.0 Black Pigment Dispersion Solution 3 40.0Black Pigment Dispersion Solution 4 40.0 Black Pigment DispersionSolution 5 Black Pigment Dispersion Solution 6 Black Pigment DispersionSolution 7 Black Pigment Dispersion Solution 8 Black Pigment DispersionSolution 9 Water-soluble Glycerol 5.0 10.0 organic solvent Diethyleneglycol 10.0 10.0 10.0 5.0 10.0 Polyethylene glycol (*1) 2.0 2.0 2.0 2.02.0 Surfactant Acetylenol EH (*2) 0.1 0.1 0.1 0.1 0.1 0.1 Ion-exchangedwater 47.9 47.9 47.9 47.9 49.9 57.9 Content of pigment [mass %] 4.0 4.04.0 4.0 4.0 3.0 Content of free polymer having hydroxyl groups [mass %]0.08 0.20 0.18 0.20 0.15 0.15 Content of polymer having hydroxyl groups[mass %] 0.36 0.32 0.28 0.32 0.36 0.32 Content of polymer [mass %] 0.360.32 0.68 0.32 0.36 0.32 Content of free polymer having hydroxylgroups/Content of pigment × 100 [%] 2.0 5.0 4.5 5.0 3.8 5.0 Content ofpolymer having hydroxyl groups/Content of pigment × 100 [%] 9.0 8.0 7.08.0 9.0 10.7 Content of free polymer having hydroxyl groups/Content ofpolymer × 100 [%] 22.2 62.5 26.5 62.5 41.7 46.9(*1) Average molecular weight: 1,000.(*2) Acetylene glycol ethylene oxide adduct (product of Kawaken FineChemicals Co., Ltd.).

TABLE 2 Ink 7 8 9 10 11 12 Coloring material Black Pigment DispersionSolution 1 Black Pigment Dispersion Solution 2 Black Pigment DispersionSolution 3 Black Pigment Dispersion Solution 4 40.0 Black PigmentDispersion Solution 5 40.0 Black Pigment Dispersion Solution 6 40.0Black Pigment Dispersion Solution 7 40.0 Black Pigment DispersionSolution 8 40.0 Black Pigment Dispersion Solution 9 40.0 Water-solubleGlycerol organic solvent Diethylene glycol 10.0 10.0 10.0 10.0 10.0 10.0Polyethylene glycol (*1) 2.0 2.0 2.0 Surfactant Acetylenol EH (*2) 0.10.1 0.1 0.1 0.1 0.1 Ion-exchanged water 49.9 49.9 49.9 47.9 47.9 47.9Content of pigment [mass %] 4.0 4.0 4.0 4.0 4.0 4.0 Content of freepolymer having hydroxyl groups [mass %] 0.15 0.17 0.19 0.15 0.05 0.30Content of polymer having hydroxyl groups [mass %] 0.60 0.36 0.64 0.360.36 0.68 Content of polymer [mass %] 0.60 0.36 0.64 0.36 0.36 0.68Content of free polymer having hydroxyl groups/Content of pigment × 100[%] 3.8 4.3 4.8 3.8 1.3 7.5 Content of polymer having hydroxylgroups/Content of pigment × 100 [%] 15.0 9.0 16.0 9.0 9.0 17.0 Contentof free polymer having hydroxyl groups/Content of polymer × 100 [%] 25.047.2 29.7 41.7 13.9 44.1(*1) Average molecular weight: 1,000.(*2) Acetylene glycol ethylene oxide adduct (product of Kawaken FineChemicals Co., Ltd.).

<Evaluation>

(Seeping Out Phenomenon)

Each of the inks obtained above was stored into an ink cartridge, andthe ink cartridge was installed at the position of the cyan ink of amodified ink jet recording apparatus “PIXUS 860i” (manufactured by CanonInc.). After a head cleaning operation was then conducted twice toconfirm that the ink has reached an ejection orifice, the main power ofthe ink jet recording apparatus was turned off. After the ink jetrecording apparatus was left to stand for 2 weeks at room temperature inthis state, the recording head was taken out of the apparatus bodywithout turning on the main power while the ink cartridge was keptinstalled, and the condition in the vicinity of the ejection orifices(nozzles) was observed through a microscope. The recording head was theninstalled again in the ink jet recording apparatus, the main power wasturned on, and the head cleaning operation was conducted once.Thereafter, a nozzle check pattern was recorded. The ink was evaluatedas to the seeping out phenomenon by the condition of the nozzles and thenozzle check pattern. The evaluation criteria of the seeping outphenomenon were as follows. The results of the evaluation are shown inTable 3.

[Evaluation Criteria of Seeping Out Phenomenon]

A: No seeping out phenomenon was observed on all nozzles, and recordingfailure was not recognized on the nozzle check pattern.

B: The seeping out phenomenon was observed on less than 1% of allnozzles, but recording failure was not recognized on the nozzle checkpattern.

C: The seeping out phenomenon was observed on 1% or more and less than5% of all nozzles, and recording failure was recognized on the nozzlecheck pattern in some of the nozzles.

D: The seeping out phenomenon was observed on 5% or more of all nozzles,and recording failure was recognized on the nozzle check pattern.

(Storage Stability)

Each of the inks obtained above was placed in a shot bottle, and theshot bottle was closely sealed with a cap and stored for one month in anoven of 60° C. After the shot bottle was then taken out of the oven andleft to stand until the temperature of the bottle was cooled to ordinarytemperature, the bottle was caused to stand with the cap being downsideto visually observe the amount and size of the solid deposited on thebottom of the bottle. The values of physical properties (viscosity,surface tension, average particle size of pigments and absorbancecharacteristics) of the ink were measured. The ink was evaluated as tostorage stability by the amount and size of the solids and the physicalproperty values of the ink. The criteria of the storage stability are asfollows. The results of the evaluation are shown in Table 3.

A: Solid is scarcely produced.

B: Solid is somewhat produced, but no particular change is recognized onthe physical property values of the ink.

C: Solid is produced in a great amount, and any of the physical propertyvalues of the ink is changed from the initial value. TABLE 3 Seeping outStorage Ink phenomenon stability Example 1 1 A B 2 2 B A 3 3 B A 4 4 B A5 5 A B 6 6 A A 7 7 A B 8 8 B B 9 9 B A 10 10 A B Comparative 1 11 A CExample 2 12 D A

(Abrasion Resistance)

Each of Inks 1 to 3, 5, 7 and 10 obtained above was stored into an inkcartridge, and the ink cartridge was installed at the position of theblack ink of a modified ink jet recording apparatus “PIXUS 860i”(manufactured by Canon Inc.). Thereafter, a solid print portion of 2cm×2 cm and characters were recorded on a recording medium (PB Paper,product of Canon Inc.) to prepare a recorded matter. Incidentally, theejection quantity per dot of the ink was within 30 ng±10%.

For the printer driver a default mode was selected.

Kind of paper: plain paper.

Print quality: standard.

Color adjustment: automatic.

After the recorded matter obtained above was left to stand for oneminute at room temperature, the recorded portion was rubbed with afinger to visually observe the degree of stain of the solid printedportion, the characters and the finger, thereby evaluating the ink as tothe scratch resistance. The criteria of the scratch resistance were asfollows. The results of the evaluation are shown in Table 4.

[Evaluation Criteria of Scratch Resistance]

A: Scraping of the recorded portions is scarcely conspicuous at bothsolid-printed area and character-printed area, and no finger was soiled.

B: The solid-printed area or character-printed area is scraped off, andthe finger was also soiled.

(Highlighter resistance)

Each of Inks 1 to 3, 5, 7 and 10 obtained above was stored into an inkcartridge, and the ink cartridge was installed at the position of theblack ink of a modified ink jet recording apparatus “PIXUS 860i”(manufactured by Canon Inc.). Thereafter, a solid print portion of 2cm×2 cm and characters were recorded on a recording medium (PB Paper,product of Canon Inc.) to prepare a recorded matter. Incidentally, theejection quantity per dot of the ink was within 30 ng±10%.

For the printer driver a default mode was selected.

Kind of paper: plain paper.

Print quality: standard.

Color adjustment: automatic.

After the recorded matter obtained above was left to stand for one hourat room temperature, the character-printed area was marked twice underan ordinary writing pressure with a yellow highlighter (Spot Writer,product of Pilot Pen Co., Ltd.), whereby the degree of stain of therecorded matter and the pen point was visually observed to evaluate theink as to the highlighter resistance. The criteria of the highlighterresistance were as follows. The results of the evaluation are shown inTable 4.

[Evaluation Criteria of Highlighter Resistance]

A: Neither blurring of the recorded portions nor stain of white portionsis observed, and the pen point is also not soiled.

B: No stain was observed on the white portions in the recorded matter,but the pen point is slightly soiled.

C: Stain was observed on the white portions in the recorded matter.

(Ejection Characteristics)

Each of Inks 1 to 3, 5, 7 and 10 obtained above was stored into an inkcartridge, and the ink cartridge was installed at the position of theblack ink of a modified ink jet recording apparatus “PIXUS 860i”(manufactured by Canon Inc.). Thereafter, a solid print portion having arecording density of 100% in a size of 2 cm×8 cm was recorded on arecording medium (PB Paper, product of Canon Inc.) with a drivefrequency varied from 100 Hz to 1 kHz, 5 kHz, 10 kHz and 15 kHz toprepare a recorded matter. Incidentally, the ejection quantity per dotof the ink was within 30 ng±10%.

For the printer driver a default mode was selected.

Kind of paper: plain paper.

Print quality: standard.

Color adjustment: automatic.

The image densities and condition of unevenness of the solid printedimages in the recorded matter obtained above, and the condition ofejection failure upon the recording were visually observed to evaluatethe ink as to the ejection characteristics. The criteria of the ejectioncharacteristics were as follows. The results of the evaluation are shownin Table 4.

[Evaluation Criteria of Ejection Characteristics]

A: No ejection failure occurs up to 15 kHz, and solid printed imagesobtained by recording at 15 kHz and 100 Hz are comparable in imagedensity and unevenness when compared with each other.

B: No ejection failure occurs up to 15 kHz, but the solid printed imageat 15 kHz is poorer in image density and condition of unevenness whencompared with the solid printed image at 100 Hz.

C: No ejection failure occurs up to 10 kHz, but the solid printed imageat 15 kHz undergoes unevenness due to the presence of nozzles ofejection failure and occurrence of dot misalignment. TABLE 4 ScratchHighlighter Ejection Ink resistance resistance characteristics Example11 1 A B A 12 2 A B A 13 3 A B A 14 5 A B B 15 7 A A C 16 10 A B A

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2005-314709, filed Oct. 28, 2005, which is whereby incorporated byreference herein in its entirety.

1. An aqueous ink comprising a polymer having hydroxyl groups and apigment, wherein a proportion of a content of a polymer that is notadsorbed on the pigment out of the polymer having hydroxyl groups to acontent of the pigment in the ink is 2.0 mass % or more, and wherein acontent (mass %) of the polymer that is not adsorbed on the pigment outof the polymer having hydroxyl groups is 0.20 mass % or less based onthe total mass of the ink.
 2. The aqueous ink according to claim 1,wherein a content (mass %) of the polymer having hydroxyl groups in theink is from 5.0% or more to 100.0% or less based on the total mass ofthe ink when a content (mass %) of the pigment in the ink is regarded as100 based on the total mass of the ink.
 3. The aqueous ink according toclaim 1, wherein the polymer having hydroxyl groups is a polyvinylalcohol type polymer.
 4. An ink jet recording method comprising ejectingan ink by an ink jet method to conduct recording on a recording medium,wherein the ink is the aqueous ink according to claim
 1. 5. An inkcartridge comprising an ink storage portion storing an ink, wherein theink is the aqueous ink according to claim
 1. 6. A recording unitcomprising an ink storage portion storing an ink and a recording headfor ejecting the ink, wherein the ink is the aqueous ink according toclaim
 1. 7. An ink jet recording apparatus comprising an ink storageportion storing an ink and a recording head for ejecting the ink,wherein the ink is the aqueous ink according to claim 1.